Something you could sing but don't (7)

I believe the answer is:

refrain

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'don't' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for refrain that I've seen before include "Hold back" , "Much repeated saying" , "Do without" , "Words repeated after verses" , "Chorus; keep away (from)" .)

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